In order to protect the lives of passengers during a traffic accident, modern vehicles are generally fitted with a protection system comprising several airbags and seat belt pretensioners, which are used to absorb the energy of a passenger released during the collision due to the accident. It is clear that such systems are even more effective when they are better adapted to the specific requirements of each passenger, i.e. to the weight and/or the size of the passenger. That is why microprocessor-controlled protection systems have been designed which provide several operational modes, allowing for example an adaptation of the instant at which airbags are deployed, the volume to which the airbags are inflated, the instant at which safety belts are released after the collision, etc, as a function of the stature of the passenger and the orientation of the passenger on the seat.
In order to enable the control microprocessor to select the optimum operational mode for a given seat occupancy status, it is therefore necessary to detect one or several parameters characterizing the occupancy status of the seat and to classify the occupancy into one of several classes, each of which is associated to a specific operational mode of the restraint system.
The detection of the occupancy parameters is commonly achieved by seat occupancy sensors, which comprise a plurality of pressure sensors distributed over the surface of the seat. The pressure sensors comprise pressure sensitive resistors, i.e. the resistance of these pressure sensors changes with the pressure applied on the sensor. The reading of the resistance values of the individual pressure sensors thus gives an indication on the pressure acting on each cell and accordingly can be related to the weight acting on the seat. Furthermore the distribution of the pressure values over the surface of the seat can be related to the size or the form of a person or an object occupying the seat.
In a very simple method for controlling the restraint system, the occupancy status is repeatedly monitored by means of one or more specific parameters of the occupancy detector, and an actual occupancy class is associated to the measured parameter. This actual occupancy class is then directly used by the microprocessor for selecting the adequate operational mode of the restraint system. Unfortunately a passenger often changes its position on the seat, thereby shifting its weight respectively its center of weight. Each movement will change the readings on the different pressure sensors so that the classification may vary arbitrarily with time.
In order to dampen the arbitrary variations of the classification, the actual class parameter can be stored into a buffer comprising several previously determined classes and a filtered class can be set to the average value of the individual stored classes. While such a filtering provides an improved classification result, this method is still not reliable enough. In fact, a single bad classification among a series of reliable classifications immediately may have an impact on the filtered class of the passenger.